SDI (Silt Density Index)

          An empirical test developed for membrane systems to measure the rate of fouling of a 0.45 micron filter pad by the suspended and colloidal particles in a feed water. This test involves the time required to filter a specified volume of feed at a constant 30 psi at time zero and then after 5 minutes, 10 minutes and 15 minutes of continuous filtration. Typical RO element warranties list a maximum SDI of 4.0 at 15 minutes for the feed water. If the SDI test is limited to only 5 or 10 minute readings due to plugging of the filter pad, the user can expect a high level of fouling for the RO. Deep wells typically have SDI’s of 3 or less and turbidities less than one with little or no pretreatment. Surface sources typically require pretreatment for removal of colloidal and suspended solids to achieve acceptable SDI and turbidity values.

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